1. Field of the Invention
The present invention relates to a speaker apparatus such as an electromagnetically coupled speaker apparatus.
2. Description of Related Art
As a sound reproducing speaker, there has been known and reduced into practice an electromagnetically coupled speaker apparatus where a magnet is interposed between a center pole portion and a plate provided at a yoke by which a magnetic circuit having an air gap between the center pole portion and the plate is constituted, a primary coil that is a drive coil is fixed to the center pole portion or the plate in the air gap of the magnetic circuit and a secondary coil constituting a short coil is arranged in the air gap of the magnetic circuit by fixing the secondary coil to a vibrating plate such that the secondary coil is opposed to the primary coil.
According to the electromagnetically coupled speaker, secondary current is induced in the secondary coil constituting a short coil by signal current flowing in the primary coil that is a drive coil and a drive force in accordance with the secondary current is caused in the secondary coil by a mutual action of the secondary current in respect of magnetic fluxes caused in the air gap portion of the magnetic circuit based on Flemming's left hand law by which the vibrating plate fixed with the secondary coil is displaced. Voice is generated by moving the vibrating plate in such a manner.
According to the electromagnetically coupled speaker, the primary coil where the signal current flows, is provided with advantages where it is excellent in heat radiating performance and is durable to large input since it is fixed to the center pole portion or the plate constructed of a magnetic material such as iron. Further, distortion can be minimized by constituting the secondary coil which constitutes a short coil by a cylindrical body of one turn constructed of a nonmagnetic electrically conductive material, for example, aluminum.
Incidentally, as a speaker, a dynamic speaker where a voice coil bobbin is arranged to fix to a vibrating plate in an air gap of a magnetic circuit and a voice coil which is a drive coil is wound around the voice coil bobbin, or the like has been reduced into practice.
According to a speaker such as an electromagnetically coupled speaker or a dynamic speaker described above, it is conceivable to reproduce voice by providing, for example, coils of several bits of digital voice signals as drive coils and by driving the respective coils by the respective bits of signals in correspondence with the respective bits of the digital voice signals.
For example, when input digital sound signals are linear quantized signals of 16 bits in an electromagnetically coupled speaker, the primary coil of the electromagnetically coupled speaker is constituted by 16 pieces of coils, the turn numbers of the respective coils are made to correspond to the weights of respective bits of input digital voice signals, the turn number of each coil is twice as much as that of a contiguous coil corresponding to a bit at an order lower than the order of the each coil by 1. Further, when bits of the input digital voice signals corresponding to the respective coils become active, currents having a constant current value are made to flow.
The drive force F of a vibration system in an electromagnetically coupled speaker, is represented by F=BLi that is a product of a secondary current i induced in the secondary coil, the density of magnetic fluxes B generated in an air gap of a magnetic circuit and a length L of the secondary coil disposed in the air gap of the magnetic circuit where the magnetic flux density B and the length L are constant, and accordingly, the drive force F of the vibration system is proportional to the secondary current i induced in the secondary coil. The secondary current i induced in the secondary coil is proportional to a product of a signal current flowing in the primary coil and the turn number (impedance) of the primary coil.
Accordingly, when the electromagnetically coupled speaker is constructed as mentioned above, the vibrating plate where the secondary coil is fixed, is displaced in one direction by an amount in proportion to the weights of the respective bits of the input digital sound signals and voice is reproduced honestly in compliance with the input digital sound signals.
In the meantime, it is necessary for a speaker device to change the sound volume. That is, in the case where the drive coil of a speaker such as an electromagnetically coupled speaker or a dynamic speaker, is directly driven by digital sound signals as described above, it is conceivable as a system of controlling the sound volume that the drive coil of the speaker is constituted by coils having a number that is larger than the number of bits of the input digital sound signals and driving coils are switched by the input digital sound signals in accordance with a desired sound volume.
For example, in respect of an electromagnetically coupled speaker, when the input digital sound signals are constituted by 16 bits, the primary coils of the electromagnetically coupled speaker is constituted by 24 pieces of coils, the turn numbers of the respective coils are changed in a geometric series as described above and in order to obtain a maximum sound volume, 16 pieces of the coils on the side of larger turn numbers are driven by the digital sound signals of 16 bits and in order to obtain a minimum sound volume, 16 pieces of coils on the side of smaller turn numbers are driven by 16 bits of the input digital sound signals.
However, according to this system, the drive coils having a number that is larger than the number of bits of the input digital sound signals, are needed, a total of the turn numbers of the drive coils are considerably increased, the structure of speaker is complicated and fabrication cost is increased. Further, the system is provided with inconvenience where the stages of sound volume control are restricted by the number of drive coils such as 8 stages according to the above-described example.
Therefore, according to the present invention, the sound volume of a speaker can finely be controlled by a speaker structure that is simple and is fabricated at a low cost in the case where drive coils of a speaker such as an electromagnetically coupled speaker are driven by digital sound signals.